Data transmission (also data communication or digital communications) is the transfer of data (a digital bit stream) over a point-to-point or point-to-multipoint communication channel. Examples of such channels are copper wires, optical fibers, wireless communication channels, storage media and computer buses. A channel may be used to convey information, for example a digital bit stream, from one or several senders (or transmitters) to one or several receivers. A channel may have a certain capacity for transmitting information, often measured by its bandwidth in Hz or its data rate in bits per second.